1. Technical Field
This disclosure relates to trimming the gain in amplifiers, including difference amplifiers, and trimming the common mode rejection ratio (CMMR) in difference amplifiers.
2. Description of Related Art
In high precision difference amplifiers, instrumentation amplifiers, and programmable gain amplifiers, gain accuracy and common mode rejection ratio (CMRR) may be limited by mismatched resistors.
SiCr resistors may be used in such circuits due to their low temp coefficients and matching properties. However, these resistors may still have mismatches between hundreds and a thousand parts per million (ppm). It may be necessary to trim these resistors to achieve only 100 ppm gain error and greater than 100 dB CMRR. However, further reducing the mismatch may require a large layout area that may be undesirable.
Laser trimming of SiCr resistors or metal link may be carried out during wafer sort. Unfortunately, laser trimming SiCr resistors may not be feasible in processes that have a planarized backend. Laser trimming can also be slow and devices under test (DUT) may still be subject to post package shift. Wafer sorting of 100 dB CMRR may also degrade to 86 dB post package due to stress. Laser trimming SiCr resistors may also not account for a temperature coefficient in both gain and CMRR. Metal link, on the other hand, may have reliability issues in addition to the above problems.
Trimming may also be performed post-package using digital switching.
However, this approach may require a dramatic increase in the complexity of the circuit, particular when the difference amplifier operates at a high voltage. These switches could also see a large signal swing that may require robust protection. An undesirable temperature coefficient in gain and CMRR may also remain. The switches may also increase input noise, leakage current, and make frequency compensation more difficult.